Initiator

ABSTRACT

An initiator includes a pair of electrode pins, a conductive header, an insulator, a bridge wire, an explosive, and capsules. A bottom wall of the inner capsule has a protrusion projecting from the bottom wall. The thickness of an end wall portion of the protrusion is determined in such a manner that the end wall portion is the thinnest among a plurality of wall portions of the inner capsule. The thicknesses of the wall portions are set so as to increase gradually from the end wall portion of the protrusion toward the side wall of the inner capsule. Therefore, when the explosive is ignited for detonation, breakage of the inner capsule occurs in the vicinity of the end wall portion of the protrusion, and flame propagation energy generated by detonation of the explosive propagates in an intended direction.

TECHNICAL FIELD

The present invention relates to an initiator to be employed in, forexample, an airbag apparatus or a seatbelt pre-tensioner, either ofwhich is to be furnished in a vehicle.

BACKGROUND ART

One of various known types of initiators is constituted by a pair ofelectrodes joined together via an insulator; a bridge wire connectedwith these two electrodes and adapted to generate heat when energized;and a capsule accommodating the bridge wire and an explosive able todetonate in response to heat generation of the bridge wire. This type ofinitiator is disclosed in, for example, Japanese Patent ApplicationLaid-Open (kokai) No. H11-301402.

In order to manufacture the above-described type of initiator compactly,the individual components of the initiator must be reduced in size,which results in a reduction in the usable amount of explosive (theamount of explosive that can be charged in the capsule). For thisreason, conceivably, when the explosive is ignited for detonation insuch a downsized conventional structure, there arises a fear that adesired level of flame propagation energy cannot be obtained in anintended direction.

DISCLOSURE OF THE INVENTION

To solve the above-mentioned problems, the present invention provides aninitiator comprising a pair of electrodes integrated together via aninsulator; a bridge wire connected between the pair of electrodes andadapted to generate heat when energized; and a bottomed tubular capsuleaccommodating the bridge wire and an explosive which detonates inresponse to heat generation of the bridge wire, wherein a recess isformed in a central portion of a bottom wall of the capsule, the recessinducing breakage when the explosive is ignited for detonation, and aplurality of concentric grooves are formed around the recess in such amanner that an inner groove has a width and a depth greater than thoseof an outer groove.

With this configuration, when the explosive is ignited for detonation,breakage of the capsule can be concentrated in the vicinity of thecentral portion of the bottom wall of the capsule, so that flamepropagation energy (pressure or flamepower) generated by detonation ofthe explosive propagates in an intended direction through the brokenportion. Therefore, even if the amount of the explosive charged in thecapsule is small, an intended magnitude of flame propagation energy canbe attained in an intended direction upon detonation of the explosive.Accordingly, it is possible to reduce the size of the initiator whilemaintaining the function of the initiator.

Moreover, a plurality of concentric grooves are formed on the bottomwall of the capsule around the recess formed in the central portion ofthe bottom wall, in such a manner that an inner groove has a width and,a depth greater than those of an outer groove. Therefore, the strengthof the bottom wall of the capsule can be increased from the centralportion of the bottom wall toward the sidewall of the capsule by meansof the plurality of concentric grooves around the recess. As a result,propagation of breakage from the central portion of the bottom walltoward the sidewall of the capsule can be reliably suppressed. Further,the strength of the bottom wall of the capsule can be increased from thecentral portion toward the sidewall of the capsule by means of a simpleconfiguration.

When the present invention is practiced, in place of the plurality ofconcentric grooves around the recess formed in the central portion ofthe bottom wall of the capsule, there can be employed a spiral groveformed in such a manner that the spiral groove starts from the recessprovided in the central portion of the bottom wall of the capsule, andthe spiral gradually decreases in width and depth toward an outercircumferential portion of the bottom wall. In this case as well, actionand effects similar to those described above can be expected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an inflator for an airbagapparatus to be mounted in a vehicle in which inflator an initiatoraccording to an embodiment of the present invention is employed;

FIG. 2 is an enlarged cross-sectional view of a main portion of theinitiator of FIG. 1;

FIG. 3 is an enlarged cross-sectional view of a main portion of an innercapsule alone, which is shown FIGS. 1 and 2;

FIG. 4 is a schematic cross-sectional view of a first modification ofthe inner capsule of FIG. 3;

FIG. 5 is a schematic cross-sectional view of a second modification ofthe inner capsule of FIG. 3;

FIG. 6 is a schematic cross-sectional view of a third modification ofthe inner capsule of FIG. 3;

FIG. 7 is a bottom plan view of the inner capsule of FIG. 6;

FIG. 8 is explanatory view illustrating transitional stages of breakageof the inner capsule of FIGS. 6 and 7; and

FIG. 9 is a schematic cross-sectional view of a fourth modification ofthe inner capsule of FIG. 3.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will now be described with thedrawings. FIG. 1 shows an inflator 10 for an airbag apparatus to bemounted in a vehicle, the inflator 10 incorporating an initiator 20according to the present invention. The inflator 10 of the presentembodiment includes a casing 11 and a breakable gas sealing lid 12mounted in an airtight manner in the casing 11. The casing 11 has a gasstorage portion 11 a, in which a high-pressure gas is enclosed, and anattachment portion 11 b, to which the initiator 20 is attached.

While in an unbroken state, the gas sealing lid 12 serves to reserve ahigh-pressure gas in the storage portion 11 a of the casing 11. Notably,when the gas sealing lid 12 is broken upon detonation of the initiator20, the high-pressure gas spurts out of the gas storage portion 11 a ofthe casing 11 toward an airbag (not shown) via an outflow opening 11 b 1formed in the attachment portion 11 b.

Meanwhile, the initiator 20 includes a plurality of components shown, onan enlarged scale, in FIG. 2; e.g., a pair of electrode pins 21 a and 21b, a conductive header 22, an insulator 23, a bridge wire 24, anexplosive (gunpowder) 25, capsules 26 a, 26 b, and 26 c, and a resinmold 27. Auxiliary components; e.g., a metallic holder 28 and a resinholder 29 are used to mount the initiator 20 in the inflator 10 as shownin FIG. 1.

One of the pair of electrode pins (hereinafter also called the firstelectrode pin) 21 a is attached to the conductive header 22, and theother electrode pin (hereinafter also called the second electrode pin)21 b is attached to the conductive header 22 via the insulator 23. Theconductive header 22 is formed of a conductive metal and has acylindrical tube that has a hole 22 a along its center axis, and anannular stepped portion 22 b on its circumference.

The insulator 23 assumes the form of a cylindrical tube having an axialinsertion hole 23 a in which the second electrode pin 21 b is tightlyfitted, whereby the second electrode pin 21 b is fixedly secured to theinsulator 23 concentrically therewith. The insulator 23, which is madeof heat- and pressure-resistant glass, is tightly fitted in the hole 22a of the conductive header 22 so as to be secured to the conductiveheader 22 concentrically therewith.

The bridge wire 24 is connected between the second electrode pin 21 band the conductive header 22; i.e., the bridge wire 24 is connecteddirectly to the second electrode pin 21 b, and indirectly to the firstelectrode pin 21 a via the conductive header 22. When energized via thefirst and second electrode pins 21 a and 21 b, the bridge wire 24generates heat to thereby cause the explosive 25 to detonate. Theexplosive 25, together with the bridge wire 24, is accommodated in aleak-proof state within the inner capsule 26 a, and the explosive 25comes in partial contact with the bridge wire 24.

The inner capsule 26 a is formed of a thin metal sheet and has abottomed tubular shape (a cup shape) whose bottom wall 26 a 1 isbreakable upon detonation of the explosive 25. The open end of the innercapsule 26 a is fitted to the circumference of the conductive header 22and fixedly joined therewith in an airtight state by welding or othermeans. As shown on an enlarged scale in FIG. 3, the bottom wall 26 a 1of the inner capsule 26 a is formed so as to have a cylindricalprotrusion which projects outward from the bottom wall 26 a 1 and whichhas a corner portion of substantially right angle.

Further, as shown in FIG. 3, a sidewall (peripheral wall) 26 a 2 of theinner capsule 26 a has a thickness t4; a first bottom portion of thebottom wall 26 a 1, which portion is continuous with the sidewall 26 a2, has a thickness t3; a second bottom portion of the bottom wall 26 a1, which portion extends (downward in FIG. 3) from the first bottomportion so as to form a peripheral wall portion of the protrusion, has athickness t2; and a third wall portion of the bottom wall 26 a 1, whichportion forms an end wall portion of the protrusion, has a thickness t1.These four thicknesses are determined so as to satisfy a relationt4>t3>t2>t1. By virtue of this geometry, when the explosive 25 isignited for detonation, breakage of the inner capsule 26 ainstantaneously starts from the end wall portion of the protrusion ofthe bottom wall 26 a 1. In order that this breakage takes placereliably, a recess A (see an imaginary line in FIG. 3) which serves toinduce breakage of the inner capsule 26 a may be formed in a centralportion of the end wall portion on an outer side thereof; and a notch(incision) B (see an imaginary line in FIG. 3) may be formed in each ofthe corners of the bottom wall 26 a 1 on an inner side thereof.

The intermediate capsule 26 b is formed of a metal sheet thicker thanthe inner capsule 26 a and has a bottomed tubular shape. A bottom wall26 b 1 of the intermediate capsule 26 b has an opening C facing the endwall portion of the protrusion of the inner capsule 26 a. The open endof the intermediate capsule 26 b is fitted to the circumference of theconductive header 22 and fixedly joined therewith by welding or othermeans. The outer capsule 26 c is an insulator cap formed of a thin resinsheet and has a bottomed tubular shape. The outer capsule 26 c isfixedly fitted to the circumference of the intermediate capsule 26 b.The resin mold 27 is formed to integrally join the individualcomponents; e.g., the first and second electrode pins 21 a and 21 b, theconductive header 22, the insulator 23, and the capsules 26 a, 26 b, and26 c.

The metallic holder 28 is formed of a metal sheet and, cooperates withthe resin holder 29 to hold the initiator 20 of FIG. 2 in the mannershown in FIG. 1. The resin holder 29 is formed through molding in orderto cover the initiator 20 assembled to the metallic holder 28. As shownin FIG. 1, the resin holder 29 is assembled to the casing 11 togetherwith the metallic holder 28.

In the thus-constructed initiator 20 of the present embodiment, thebottom wall 26 a 1 of the inner capsule 26 a is formed so as to have aprotrusion projecting therefrom; the thickness of the end wall portionof the protrusion is determined such that the end wall portion is thethinnest among the wall portions of the inner capsule 26 a; and thethicknesses of the wall portions are set so as to increase graduallyfrom the end wall portion of the protrusion toward the side wall 26 a 2of the inner capsule 26 a. Therefore, when the explosive 25 is ignitedfor detonation, breakage of the inner capsule 26 a can be concentratedin the vicinity of the end wall portion of the protrusion, and flamepropagation energy (pressure or flamepower) generated by detonation ofthe explosive 25 propagates in an intended direction (leftward; i.e.,toward the gas sealing lid 12 in FIG. 1).

Further, because the flame propagation energy generated by detonation ofthe explosive 25 can be guided to the end wall portion of the protrusionof the inner capsule 26 a, the magnitude of the flame propagation energyin the above-described intended direction can be increased. Stillfurther, the intermediate and outer capsules 26 b and 26 c are placedover the inner capsule 26 a to constitute a capsule assembly having alayered structure such that an intended strength can be attainedreliably at a bottom wall portion extending from a broken portion of thecapsule assembly to the sidewall thereof. Therefore, progress of thebreakage from the broken portion toward the sidewall of the capsuleassembly can be restricted reliably.

Consequently, in the initiator 20 of the present embodiment, even if theamount of the explosive 25 charged in the inner capsule 26 a is small,an intended magnitude of flame propagation energy can be attained in anintended direction upon detonation of the explosive 25. Accordingly, theinitiator 20 can be assembled compactly while maintaining the properfunction (i.e., the function of breaking the gas sealing lid 12 in theinflator 10) of the initiator 20.

In the above-described embodiment, as shown in FIGS. 1 to 3, the bottomwall 26 a 1 of the inner capsule 26 a of the initiator 20 has aprotrusion projecting from the center of the bottom wall 26 a 1.However, as schematically shown in FIG. 4, the protrusion projectingfrom the bottom wall 26 a 1 of the inner capsule 26 a may be offset apredetermined distance from the center of the bottom wall 26 a 1. Inthis first modification, the sidewall 26 a 2 of the inner capsule 26 ahas a thickness t4; a first wall portion of the bottom wall 26 a 1,which portion is continuous with the sidewall 26 a 2, has a thicknesst3; a second wall portion of the bottom wall 26 a 1, which portionextends (downward in FIG. 4) from the bottom wall 26 a 1 so as to form aperipheral wall portion of the protrusion, has a thickness t2; and athird wall portion of the bottom wall 26 a 1, which portion forms an endwall portion of the protrusion, has a thickness t1. These fourthicknesses are determined so as to satisfy a relation t4>t3≧t2>t1.Therefore, in the first modification, the same operation and results asin the above-described embodiment can be expected, and the direction(i.e., the intended direction) of propagation of the flame propagationenergy can be adjusted by adjustably setting the above-described offsetdistance.

Further, in the above-described embodiment, as shown in FIGS. 1 to 3,the bottom wall 26 a 1 of the inner capsule 26 a in the initiator 20 hasa protrusion projecting from the center of the bottom wall 26 a 1 sothat breakage of the inner capsule 26 a is induced (i.e., initiated) atthe central portion of the bottom wall 26 a 1 upon detonation of theexplosive 25 and that progress of the breakage from the central portiontoward the sidewall 26 a 2 is restricted. However, the initiator 20 maybe constructed according to a second, a third, or a fourth modificationas schematically shown in FIG. 5, FIGS. 6 to 8, or FIG. 9, respectively.

In the second modification of FIG. 5, a large-size recess A1 serving toinduce breakage is formed in a central portion of the bottom wall 26 a 1of the inner capsule 26 a, and a plurality of small-size recesses A2 areformed in the bottom wall 26 a 1 in a varying density decreasinggradually from the central portion of the bottom wall 26 a 1 toward theperipheral portion thereof (i.e., the sidewall 26 a 2 of the innercapsule 26 a). In the present modification, because the plurality ofsmall-size recesses A2 are formed in the above-described distribution,the strength increases gradually from the central portion of the bottomwall 26 a 1 toward the peripheral portion thereof.

In the third modification shown in FIGS. 6 to 8, a recess A serving toinduce breakage is formed in a central portion of the bottom wall 26 a 1of the inner capsule 26 a, and a plurality of concentric annular grooves(hereinafter also called outer and inner annular grooves) S1 and S2 areformed around the recess A. The outer annular groove S1 has a width w1and a depth d1. Meanwhile, the inner annular groove S2 has a width w2(w2>w1) and a depth d2 (d2>d1). By virtue of this geometry, in thepresent modification as well, the strength increases gradually from thecentral portion of the bottom wall 26 a 1 toward the peripheral portionthereof. Consequently, in the present modification, as shown in FIG. 8,the bottom wall 26 a 1 of the inner capsule 26 a is broken graduallyfrom the central portion of the bottom wall 26 a 1 toward the sidewall26 a 2 of the inner capsule 26 a in response to detonation of theexplosive 25.

In the fourth modification of FIG. 9, a recess A serving to inducebreakage is formed in a central portion of the bottom wall 26 a 1 of theinner capsule 26 a, and a spiral groove S is formed in the bottom wall26 a 1 around the recess A. In the present modification, the spiralgroove S gradually decreases in both width and depth from the centralportion of the bottom wall 26 a 1 toward the peripheral portion thereofand, therefore, the strength increases gradually from the centralportion of the bottom wall 26 a 1 toward the peripheral portion thereof,as in the above-described embodiment and modifications.

In the above-described embodiment, the initiator 20 of the presentinvention is employed in the inflator 10 which is for use in an airbagapparatus to be mounted in a vehicle and is equipped with the casing 11and the gas sealing lid 12. Alternatively, the initiator of the presentinvention may be employed in another type of inflator (e.g., an inflatorequipped with a casing containing a gas generating agent which generatesgas upon combustion) or another type of apparatus (e.g., a seatbeltpre-tensioner). In addition, in carrying out the present invention, twoor more of the features of the above-described embodiment andmodifications may be combined.

1-14. (canceled)
 15. An initiator comprising a pair of electrodesintegrated together via an insulator; a bridge wire connected betweenthe pair of electrodes and adapted to generate heat when energized; anda bottomed tubular capsule accommodating the bridge wire and anexplosive which detonates in response to heat generation of the bridgewire, wherein a recess is formed in a central portion of a bottom wallof the capsule, the recess inducing breakage when the explosive isignited for detonation, and a plurality of concentric grooves are formedaround the recess in such a manner that an inner groove has a width anda depth greater than those of an outer groove.
 16. An initiatorcomprising a pair of electrodes integrated together via an insulator; abridge wire connected between the pair of electrodes and adapted togenerate heat when energized; and a bottomed tubular capsuleaccommodating the bridge wire and an explosive which detonates inresponse to heat generation of the bridge wire, wherein a recess isformed in a central portion of a bottom wall of the capsule, the recessinducing breakage when the explosive is ignited for detonation, and aspiral grove is formed in such a manner that the spiral groove startsfrom the recess, and the spiral gradually decreases in width and depthtoward an outer circumferential portion of the bottom wall.